In the last ten years, two-dimensional infrared spectroscopy has become an important technique for studying molecular structures and dynamics. We report the implementation of heterodyne detected two-dimensional sum-frequency generation (HD 2D SFG) spectroscopy, which is the analog of 2D infrared (2D IR) spectroscopy, but is selective to noncentrosymmetric systems such as interfaces. We implement the technique using mid-IR pulse shaping, which enables rapid scanning, phase cycling, and automatic phasing. Absorptive spectra are obtained, that have the highest frequency resolution possible, from which we extract the rephasing and nonrephasing signals that are sometimes preferred. Using this technique, we measure the vibrational mode of CO adsorbed on a polycrystalline Pt surface. The 2D spectrum reveals a significant inhomogenous contribution to the spectral line shape, which is quantified by simulations. This observation indicates that the surface conformation and environment of CO molecules is more complicated than the simple "atop" configuration assumed in previous work. Our method can be straightforwardly incorporated into many existing SFG spectrometers. The technique enables one to quantify inhomogeneity, vibrational couplings, spectral diffusion, chemical exchange, and many other properties analogous to 2D IR spectroscopy, but specifically for interfaces. multidimensional spectroscopy | vibrational spectroscopy | surface-sensitive | Pt catalysis | CO monolayer M olecular spectroscopies are some of the best tools for studying structures and dynamics. Two particularly useful variants are sum-frequency generation (SFG) and two-dimensional infrared (2D IR) spectroscopy. SFG spectroscopy provides a vibrational spectrum of molecular systems that lack an inversion center (1), and so has become a valuable tool for probing interfaces because no signal arises from the bulk. SFG spectroscopy has helped reveal the surface structure of liquids, characterize the surfaces of materials, and probe membrane proteins, to name only a few applications (2-5). 2D IR spectroscopy is also a vibrational spectroscopy, although not interface specific. 2D IR spectroscopy spreads the infrared spectrum into a second coordinate so that coupled vibrational modes are correlated by cross peaks, vibrational dynamics quantified by 2D line shapes, and energy transfer or chemical exchange revealed from peak intensities (6-8), in addition to many other capabilities not possible with linear one-dimensional (1D) vibrational spectroscopies like SFG spectroscopy. 2D IR spectroscopy is now being used to study protein structure and dynamics, solvent dynamics, charge transfer in semiconductors, and many other processes (9-12). These two techniques might be considered the core technologies of modern infrared spectroscopy.In this article, we combine the surface sensitivity of SFG spectroscopy with the multidimensional capabilities of 2D IR spectroscopy in a technique that we call heterodyne detected (HD) 2D SFG spectroscopy. With this technique, one obtain...

Sum-frequency generation (SFG) spectroscopy is a ubiquitous tool in the surface sciences. It provides infrared transition frequencies and line shapes that probe the structure and environment of molecules at interfaces. In this article, we apply techniques learned from the multidimensional spectroscopy community to SFG spectroscopy. We implement balanced heterodyne detection to remove scatter and the local oscillator background. Heterodyning also separates the resonant and nonresonant signals by acquiring both the real and imaginary parts of the spectrum. We utilize mid-IR pulse shaping to control the phase and delay of the mid-IR pump pulse. Pulse shaping allows phase cycling for data collection in the rotating frame and additional background subtraction. We also demonstrate time-domain data collection, which is a Fourier transform technique, and has many advantages in signal throughput, frequency resolution, and line shape accuracy over existing frequency domain methods. To demonstrate time-domain SFG spectroscopy, we study an aryl isocyanide on gold, and find that the system has an inhomogeneous structural distribution, in agreement with computational results, but which was not resolved by previous frequency-domain SFG studies. The ability to rapidly and actively manipulate the mid-IR pulse in an SFG pules sequence makes possible new experiments and more accurate spectra.

Surface-bound polypeptides and proteins are increasingly used to functionalize inorganic interfaces such as electrodes, but their structural characterization is exceedingly difficult with standard technologies. In this paper, we report the first two-dimensional sum-frequency generation (2D SFG) spectra of a peptide monolayer, which is collected by adding a mid-IR pulse shaper to a standard femtosecond SFG spectrometer. On a gold surface, standard FTIR spectroscopy is inconclusive about the peptide structure because of solvation-induced frequency shifts, but the 2D lineshapes, anharmonic shifts, and lifetimes obtained from 2D SFG reveal that the peptide is largely α-helical and upright. Random coil residues are also observed, which do not themselves appear in SFG spectra due to their isotropic structural distribution, but which still absorb infrared light and so can be detected by cross-peaks in 2D SFG spectra. We discuss these results in the context of peptide design. Because of the similar way in which the spectra are collected, these 2D SFG spectra can be directly compared to 2D IR spectra, thereby enabling structural interpretations of surface-bound peptides and biomolecules based on the well-studied structure/2D IR spectra relationships established from soluble proteins.

The composition dependence of the phase behavior and viscoelasticity of polyelectrolyte complex coacervates was investigated in a series of model acrylamide copolymers with varying charge density and nonionic comonomer hydrophobicity. The polymers, which were prepared by postpolymerization functionalization of poly­(N-acryloxy succinimide), were prepared with charged tertiary amine or carboxylic acid side chains on between 60 and 100% of the monomers, with the balance composed of either hydrophilic (acrylamide) or hydrophobic (butyl acrylamide) functional groups. Characterization by optical turbidity and small-amplitude oscillatory shear rheology revealed that as the charge density of the polymers decreased, the transition salt concentration and concentration of polymer in the coacervates also decreased, while the relaxation dynamics sped up. Interestingly, these properties depended strongly on the charge density but were relatively insensitive to the hydrophobicity of the nonionic comonomer. This counterintuitive result, which we attribute to the relatively weak potential of mean force for the interaction of short hydrated alkyl chains, suggests that choosing hydrophobic groups with relatively strong interactions will be critical for designing coacervates with targeted material properties.

We use nonlinear 2D IR spectroscopy to study TiO(2) nanocrystalline thin films sensitized with a Re dye. We find that the free electron signal, which often obscures the vibrational features in the transient absorption spectrum, is not observed in the 2D IR spectra. Its absence allows the vibrational features of the dye to be much better resolved than with the typical IR absorption probe. We observe multiple absorption bands but no cross peaks in the 2D IR spectra, which indicates that the dyes have at least three conformations. Furthermore, by using a pulse sequence in which we initiate electron transfer in the middle of the infrared pulse train, we are able to assign the excited state features by correlating them to the ground state vibrational modes and determine that the three conformations have different time scales and cross sections for electron injection. 2D IR spectroscopy is proving to be very useful in disentangling overlapping structural distributions in biological and chemical physics processes. These experiments demonstrate that nonlinear infrared probes are also a powerful new tool for studying charge transfer at interfaces.